The present invention generally relates to patch antennas, and more particularly to an improved patch antenna having improved physical stability and performance.
One known form of antenna, known as a patch antenna, utilizes a planar patch of conductive material disposed parallel to a ground plane and separated therefrom by a dielectric layer. A feed is provided to communicate electromagnetic energy to or from the patch, and a shorting pin shorts the center area of the patch to the ground plane through the dielectric so as to fix the center of the patch at a ground potential. Patch antennas may be inexpensively manufactured and may be readily formed into low cost, light weight phased array antenna systems.
One difficulty with patch antennas is their narrow bandwidth. In patch antennas previously reported, the bandwidth of the antenna may be as narrow as one or two percent of the center frequency of the antenna. Although in theory the bandwidth of the antenna should increase as the dielectric layer separating the patch from the ground plane is increased in thickness, the actual results produced by increasing the dielectric thickness have fallen short of theory. Increasing the dielectric thickness actually further narrowed the bandwidth of the antenna, while also substantially increasing the mismatch between the antenna and the feed, producing inefficient operation.
In addition, by increasing the dielectric thickness from the patch to the ground plane, the stability of the antenna patch decreases.
Operation of the patch antenna in an environment having strong electron fluxes can also be problematic for patch antennas which utilize a dielectric between the patch and the ground plane. Antennas that avoid (or at least minimize) dielectric materials are preferable in such environments (e.g., aboard orbiting spacecraft), however, the dielectric material between the patch and the ground plane is often utilized to support the patch. As such, by minimizing, preferably eliminating the dielectric between the patch and the ground plane, the stability of the patch itself becomes problematic, especially under such extreme conditions as launching a spacecraft.
Dielectric materials utilized in patch antennas may include substrates doped with carbon powder to mitigate against the effects of electron charging. Carbon doping of patch substrate materials is inherently complicated and is expensive to develop and qualify. While carbon-doping in principle prevents static charge build-up, it reduces the strength of adhesives and increases RF losses. Additionally, performance (both electrical and structural) is highly sensitive to material process variations and assembly techniques. Carbon-doping subtly changes the permittivity of materials thus requiring RF characterization of the material in order to design a suitable antenna element.
Another consideration is the relative thermal expansion coefficients of the metal patch and the dielectric material. As the patch expands and contracts at a different rate than the dielectric material, the frequency and other characteristics of the antenna also change and often difficult to predict.
Accordingly, there remains a need for patch antennas which do not suffer from the complications brought on by use of a dielectric material between the patch and the ground plane, and which have the mechanical stability to survive hostile environments including spacecraft liftoff.